Vehicles have been developed to perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
In some instances, a driver may have a change-of-mind while the engine is being shut down (e.g., still spinning down) and may wish to immediately restart the engine. In engines with an automatic transmission including a torque converter, the drop and subsequent sudden increase in engine speed during the change-of-mind engine restart can create a negative torque spike at the torque converter output, which is experienced as a clunk or bump by the vehicle operator. As such, the drive feel and engine startability is reduced.
Thus in one example, the above issue may be at least partly addressed by a method of controlling an engine coupled to a transmission. In one embodiment, the method comprises, during an automatic engine idle-stop, operating the transmission in a first, higher gear, and in response to a driver change-of-mind restart, shifting the transmission to a second, lower gear. Herein, the first higher gear may have a lower gear ratio enabling lower torque multiplication while the lower gear may have a higher gear ratio enabling higher torque multiplication.
In another embodiment, the method comprises, in response to an automatic engine idle-stop, tying up the transmission to a transmission case, and in response to a driver change-of-mind restart, releasing the transmission from the tie-up. In this way, a transmission output torque may be reduced during an engine idle-stop when the engine is spinning towards rest. Further, if the driver changes his mind and wishes to restart the engine before the engine has comes to rest, the torque reduction may be decreased to restore driveline torque while reducing the effects of the change-of-mind restart torque spike.
For example, during an engine idle-stop, the engine may be deactivated, and while the engine is spinning to rest (but before the engine has stopped), the transmission may be shifted (e.g., upshifted) to a higher gear (such as a transmission second gear). Additionally, or optionally, one or more transmission clutches may be engaged to lock the transmission to a transmission case. If the driver changes his mind and wishes to restart the engine before the engine has stopped, and while the engine speed is still above a threshold speed, the transmission may be downshifted to a lower gear (such as the transmission first gear) and/or the clutch pressure may be adjusted to unlock the transmission. If the driver changes his mind below the threshold speed, the downshifting and/or unlocking may be delayed until the engine has come to rest, restarted with starter motor assistance speed, and brought to the threshold speed.
In this way, the transmission output torque may be modulated during an engine idle-stop and a subsequent driver change-of-mind restart to reduce a negative torque spike experienced during the restart. By reducing the negative torque spike, clunks or bumps experienced by a vehicle operator, during a restart responsive to a driver's change-of-mind, can be reduced to improve the quality of the restart.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.